To be more precise, one would like to know how many asymptotically flat solutions of the field equations actually exist, and whether this class of solutions contains the physically interesting ones which correspond to radiative isolated gravitating systems. The only possible avenue to answering questions of existence consists of setting up appropriate initial value problems and proving existence theorems for solutions of the Einstein equations subject to the appropriate boundary conditions.
The conventional Cauchy problem which treats the Einstein equation as a second order partial differential equation for the metric field is already rather complicated by itself (see e.g. the review by Choquet-Bruhat and York ). But to obtain statements of the type mentioned above is further complicated by the fact that in order to discuss the asymptotic fall-off properties of solutions one would need to establish global (long time and large distance) existence together with detailed estimates about the fall-off behaviour of the solution.
The geometric characterization of the asymptotic conditions in terms of the conformal structure suggests to discuss also the existence problem in terms of the conformal structure. The general idea is as follows: Suppose we are given an asymptotically flat manifold which we consider as being embedded into an appropriate conformally related unphysical space-time. The Einstein equations for the physical metric imply conditions for the unphysical metric and the conformal factor relating these two metrics. It turns out that one can write down equations which are regular on the entire unphysical manifold, even at those points which are at infinity with respect to the physical metric. Existence of solutions of these ``regular conformal field equations'' on the conformal manifold then translate back to (semi-)global results for asymptotically flat solutions of the field equations in physical space. This approach towards the existence problem has been the programme followed by Friedrich since the late 1970's.
In this section we will discuss the conformal field equations which have been derived by Friedrich, and the various subproblems which have been successfully treated by using the conformal field equations.
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